The present application is a U.S. non-provisional application based upon and claiming priority from Russian Patent Application No. 99121965, which is hereby incorporated by reference.
The invention concerns a method for purifying acetone obtained from cumene by a combination of rectification and chemical processing to remove aldehyde impurities in order to produce high-quality commercial-grade acetone.
In a known method (U.S. Pat. No. 4,626,600) for obtaining acetone from cumene, cumene is oxidized with air to produce an intermediate cumene hydroperoxide, which is then decomposed with sulfuric acid to give a crude acetone-phenol mixture containing a number of impurities. This method is described, for example, in U.S. Pat. No. 4,626,600. The crude acetone-phenol mixture then undergoes rectification to separate a low-boiling xe2x80x9ccrude acetone fractionxe2x80x9d containing acetone, water, cumene, aldehydes and alpha-methylstyrene, and a xe2x80x9ccrude phenol fractionxe2x80x9d containing phenol and high-boiling compounds such as acetophenone and resins. The crude acetone fraction is further purified by rectification and/or chemical processing to produce commercial-grade acetone.
It is known that the xe2x80x9ccrude acetone fractionxe2x80x9d referred to above contains small amounts of aliphatic aldehydes, olefin and carbonyl impurities, specifically acetaldehyde, propionaldehyde, isobutylaldehyde, mesityl oxide, diacetone alcohol and hydroxyacetone, which are formed as by-products in the cumene hydroperoxide decomposition stage described above. Using only ordinary distillation technology to purify this fraction of the crude acetone is inefficient, since these aldehydes and the olefin impurities remain in the purified product, lowering its purity and quality. Therefore, to obtain a high-purity product not containing acetone aldehydes the purification methods must be improved by the following methods.
U.S. Pat. No. 4,620,901 describes a special technology for an extractive distillation, using dimethylformamide as a selective solvent to remove the aldehydes to a low level. But this method is made complicated by the fact that it requires expensive distillation equipment and produces acetone of unsatisfactory quality containing traces of solvent as an impurity that make the acetone unusable by most final end users.
U.S. Pat. No. 5,567,853 likewise describes a method which requires complex technology for extractive distillation by reacting the acetone fraction with a glycol solution containing compounds of alkali metals. But this method uses expensive solvents that can be regenerated only by using an additional distillation column, and the method produces a commercial acetone having a purity of only 98.3% w/w, which is not suitable for users of high-purity acetone, for example in the production of polycarbonate plastic.
A number of methods are known for chemical treatments involving alkali metal hydroxides (See, e.g., U.S. Pat. No. 4,722,769 and U.S. Pat. No. 4,340,447) to carry out the aldol condensation of low-boiling aldehydes that result in their converting to high-boiling aldol derivatives that are later removed from the acetone by ordinary distillation. The problem with this technology is that the aldol derivatives formed are thermally unstable and decompose in the boiler of the distillation column, liberating low-boiling aldehydes that penetrate to the top of the column and contaminate the acetone. In the method described in U.S. Pat. No. 4,340,447, a special distillation procedure with sidebar distillation is used to avoid this problem. This procedure complicates and interferes with control of the process, and in certain cases the acetone produced is not aldehyde-free, and therefore not suitable for an end use in polycarbonates.
One goal of the invention was to design a method for purifying acetone that would produce a higher quality product.
This invention represents a reliable, simple and economical process for removing aldehydes and olefin impurities from acetone by oxidizing the impurities present in the crude acetone fraction using an oxidizing agent added in an amount such that the acetone itself is not adversely affected in the production of high-quality aldehyde-free commercial-grade acetone.
In the method of the invention, the crude acetone obtained from the process for manufacturing Cumene-Phenol, which contains acetone, water, aldehydes, cumene and alpha-methylstyrene, is fed to a set-up for purifying acetone consisting of two rectifying columns. In the first rectifying column, most of the low-boiling impurities of the acetone, including various aldehyde contaminants, are evaporated and removed as overhead product. All the remaining constituents, including the acetone, are removed from the column bottom and are fed to the second rectifying column. In the second rectifying column the acetone is purified of water and other high-boiling impurities and is removed in the form of distillate, while the water, cumene, alpha-methylstyrene and other high-boiling constituents are removed from the column""s distilling flask. In the method of the invention, an alkaline agent and an oxidizing agent are added separately or together to the feed of one or both of the rectifying columns. The alkaline agent and oxidizing agent can be added virtually anywhere in the process in the second column or upstream, but it is more efficient to add both agents somewhere before the second rectification column.
The acetone purification process is preferably accomplished at an alkaline to oxidizing agent ratio of 1:0.01-1:100 in the first column feed, with the oxidizing agent being supplied at a rate of 0.05-50 g per 1000 ml of the first column feed. The oxidizing reagent is preferably fed in the form of a 0.1-20% aqueous solution.
The oxidizing agent used may be hydrogen peroxide or another inorganic oxidizing agent, including such inorganic peroxide compounds as KMnO4, etc.
In a number of cases the base and the oxidizing agent are mixed prior to distillation.
A distinguishing feature of this invention is the special chemical treatment during the distillation of the crude acetone fraction with a two-component mixture consisting of an oxidizing and an alkaline agent. The two reagents used in the chemical treatment may be added separately or mixed, either to one of the rectifying columns or to both rectifying columns, using different places for feeding them, to optimize the oxidizing reactions and to remove the aldehyde impurities in the production of high-quality commercial-grade acetone, without negatively affecting the quality of the acetone by peroxidation. In a most preferred embodiment, both the oxidizing and alkaline agent are added to both rectifying columns. The alkaline agent and the oxidizing agent may be added either to the feed stream before it enters the columns or to the liquid in the sumps of the columns. The oxidizing agents used are hydrogen peroxide, sodium peroxide, potassium permanganate and sodium permanganate. The alkaline reagents used are alkaline metal hydroxides and carbonates. These reagents can be mixed together in advance or they can be mixed inside the distillation column.
The essence of the method, as compared to known methods, consists in that, when the combined feed of the oxidizing and the alkaline reagents are present in the selected ratio, the optimal removal of aldehydes and unsaturated impurities is achieved with no harmful side-reactions for the acetone. In the present invention the ratio of the alkaline reagent to the oxidizing reagent achieved in the rectifying column should be held within a weight ratio range of alkali:oxidant from 1:0.1 to 1:100. The oxidizing agent is preferably fed at a rate of 0.05-50 gram per 1000 ml of crude acetone fraction feed in the form of a 0.1-20% w/w aqueous solution. The oxidizing agent can be added to the feed into one and/or both columns, and also to the top and/or bottom part of at least one of the rectifying columns. This chemical oxidation treatment theoretically converts the low-boiling compounds and unsaturated and carbonyl-type impurities into their high-boiling derivatives, which are water-soluble and stable to thermal decomposition. These high-boiling derivatives go into the flask of the second rectifying column and can be easily removed from the acetone with the bottom product without being converted back into aldehydes during the distillation process.
The xe2x80x9cpermanganate testxe2x80x9d (permanganate time test, an oxidation test using a solution of potassium permanganate) is widely used as an analytical test for determining the total aldehydes and other reducing impurities present in commercial-grade aldehyde. The method calls for adding a small quantity of potassium permanganate to a sample of acetone and measuring the time required for the color to dissipate. A longer color dissipation time (the permanganate time) indicates a lower content of reducing substances in the sample and a higher quality of the acetone. Most of the acetone sold on the market must have a minimum permanganate time of 2 hours, but this may be difficult to achieve if the manufacturing plant is overloaded and is operating at above design capacity. As a result, typical permanganate times range from 0.5 hour to 3 hours for aldehyde contents of more than 50 ppm in the commercial-grade acetone. If the time is longer than 5 hours, the aldehyde content is below 10 ppm and the quality of the acetone is considered to be excellent.
In addition to permanganate time, there are other important indicators of the quality of commercial-grade acetone, including water content and content of diacetone alcohol, which must be held at  less than 0.3% w/w and  less than 30 ppm, respectively. It is important to note that these quality indicators are not violated when the proposed technology of the present invention is used to remove aldehydes.